In a wind turbine the kinetic energy of the wind is used for the purpose of producing rotational movement in a rotor. This rotational movement is transmitted via a drive train to a generator which generates electrical energy from the rotational energy.
Due to the transmission of force from the rotor to the generator and due to further influences (see below), the drive train and the wind turbine components surrounding it are exposed to a multiplicity of forces, which result in oscillations, i.e. vibrations in the drive train. In this context, it is possible essentially to distinguish between axial oscillations, which therefore propagate along the axis or plurality of axes of the drive train, and radial oscillations, which travel transversely relative to this axis or axes. Such oscillations have a number of causes:
Firstly, external factors such as e.g. the wind speed, the wind direction, the external temperature or turbulence play a role. These factors of influence exert forces on the rotor, not all of which act in an axial direction of the drive train or the axis of the rotor. This results in mainly low-frequency oscillations of up to approximately 10 Hz in the drive train.
Secondly, a complex overall system consisting of numerous mechanically-interconnected components works during operation of the wind turbine. For example, provision is often made for a speed-transforming gear which transforms the relatively slow rotation of the rotor or a first shaft of the drive train into a faster rotation of a second shaft of the drive train. Vibrations (i.e. torsional oscillations) automatically occur in this gear due to the connection of the two drive train components by means of toothed gearwheels or other elements for the transmission of force. Torsional forces are also transmitted from the gear onto the machine housing, i.e. onto the internal structure of the cabin. Moreover, the gear also produces axial oscillations, mainly in the high-frequency range above 10 Hz.
Thirdly, pitch and yawing moments are produced by constraining forces in connections between the cabin and components that are mounted in the cabin. These constraining forces occur as a result of the assembly, as a result of weight distribution, and due to excitation of the natural frequencies of the drive train. The pitch and yawing moments in the drive train produce forces which act on the subfloor in the cabin of the wind turbine and can damage this. The natural frequency of an individual component is dependent on its weight and/or its inertia in this case. By combining a system of components during the assembly, new system characteristics and hence new natural frequencies are produced.
Further oscillations can be induced as a result of a second shaft being arranged, relative to the rotor, behind such a speed-transforming gear, wherein a braking device is attached to said second shaft and wherein said second shaft leads towards the generator via a coupling in a posterior region of the cabin. This coupling can be used to realize e.g. a compensation of levels in the direction of the generator, this being mounted lower or higher than the speed-transforming gear. This coupling can also cause oscillations during operation.
The oscillations and force effects in the region of the drive train as summarized here in the form of an overview represent a problem during operation of the wind turbine, because they can significantly reduce the service life of the wind turbine as a whole, or individual components thereof, and/or permanently jeopardize their functionality. In particular, high frequency oscillations above approximately 10 Hz can cause significant damage at high amplitudes, primarily in the gear, in the generator and to the cabin of the wind turbine. They often continue along the whole drive train and can even be amplified by the transformation in a gear. The VDI specification VDI 3834, whose disclosure contents are explicitly considered to be part of the present application, sets forth the principles for the measurement and evaluation of mechanical oscillations of wind turbines and their components. It contains inter alia limits that should as far as possible not be exceeded for loads caused by vibrations.
The cited oscillations and forces can be equalized by means of various countermeasures, such that as far as possible no constraining forces are transmitted from the gear or the drive train onto the cabin. For example, the drive train is currently mounted elastically on the housing of the cabin. This mounting is effected e.g. by means of a three-point or four-point mounting, which therefore comprises an elastic sprung connection between the subfloor and the drive train or the speed-transforming gear at three or four points of the drive train. In this context, the drive train can be fully or partially surrounded at at least one point along its longitudinal course, such that the drive train is stabilized both laterally and upwards. Axially soft elastomers can be used as rubber dampers for such bearings, e.g. in the form of elastomer bushes which form the contact between the gear or a drive train component and the respective bearing support or the respective bearing ring.
A three-point mounting can comprise e.g. a main bearing and a gear support: the main bearing features a bearing ring which encloses a shaft of the drive train, i.e. a drive train component. The main bearing therefore absorbs both axial and radial forces. The gear support partially encloses a speed-transforming gear from both sides, i.e. in a horizontal direction and transversely relative to the axis of the drive train. It is so designed as to be axially mobile and therefore also absorbs torsional forces. This bearing is also used for the equalization of both pitch and yawing moments. A four-point mounting comprises a second bearing, which encompasses a shaft of the drive train and therefore offers the advantage of increased system stability due to additional absorption of axial forces.
In addition to this passive equalization of oscillations by means of bearings, provision can also be made for active oscillation damping. To this end, provision can be made at the bearings to exert active forces on the drive train or on other wind turbine components connected to the drive train, which active forces counteract the oscillations of the drive train or the components connected to the drive train. However, such active damping components require additional structural space within the cabin of the wind turbine, as well as being expensive to provide and maintenance-intensive. A further type of active oscillation damping is effected by means of converters, i.e. electronic components. By selectively regulating a converter, it is possible to decrease or increase loads from the generator side. Control of the converter for the purpose of oscillation damping is therefore possible and is currently also realized; however, it reduces the efficiency of the energy production and also introduces an additional factor of influence into the control of the converter.